U.S. patent number 6,914,513 [Application Number 10/288,940] was granted by the patent office on 2005-07-05 for materials system for low cost, non wire-wound, miniature, multilayer magnetic circuit components.
This patent grant is currently assigned to Electro-Science Laboratories, Inc.. Invention is credited to Alvin H. Feingold, Merrill R. Heinz, Cornelius Y. D. Huang, Michael Alan Stein, Richard L. Wahlers.
United States Patent |
6,914,513 |
Wahlers , et al. |
July 5, 2005 |
**Please see images for:
( Certificate of Correction ) ** |
Materials system for low cost, non wire-wound, miniature,
multilayer magnetic circuit components
Abstract
This invention describes materials system and processing
conditions for manufacturing magnetic circuit components such as
induction coils and transformers that are non wire-wound, miniature
in size and, have a low manufacturing cost. The materials system of
this invention is comprised of: (1) Low Temperature Cofire Ceramic
(LTCC) tapes or thick film pastes of ferromagnetic ceramics with a
20 to 750 range of magnetic permeability to form the magnetic core
of the components, (2) Thick film buried silver conductor paste to
form the planar induction coils on individual magnetic layers, (3)
Thick film via-fill silver conductor paste to interconnect two or
more of the planar induction coils through the thickness of the
magnetic layers, (4) Thick film silver solderable top layer
conductor paste compatible with the ferrite and, (5) Thick film
dielectric paste with low magnetic permeability to redirect the
magnetic flux for enhancing the magnetic coupling coefficient and
to insulate the silver conductors for enhancing the dielectric
breakdown voltage. The key characteristics of the materials system
of this invention that facilitate manufacture of low cost non
wire-wound, miniature magnetic circuit components are: (1) Mutual
compatibility essential for either of the techniques, the cofire
technique or the sequential technique, used for manufacturing
multilayer hybrid microelectronic components, (2) Complementary
thermo-physical properties such as shrinkage and thermal expansion
coefficient essential for manufacturing flat multilayer magnetic
components, (3) Magnetic components with magnetic coupling
coefficients greater than 0.95 under optimal processing conditions
and, (4) Magnetic components with dielectric breakdown voltage
greater than 500V/mil under optimal processing conditions.
Inventors: |
Wahlers; Richard L.
(Churchville, PA), Huang; Cornelius Y. D. (Blue Bell,
PA), Feingold; Alvin H. (West Chester, PA), Heinz;
Merrill R. (Devon, PA), Stein; Michael Alan (King of
Prussia, PA) |
Assignee: |
Electro-Science Laboratories,
Inc. (King of Prussia, PA)
|
Family
ID: |
34703910 |
Appl.
No.: |
10/288,940 |
Filed: |
November 6, 2002 |
Current U.S.
Class: |
336/233; 336/177;
336/200; 336/219; 336/232 |
Current CPC
Class: |
C04B
35/265 (20130101); C04B 35/6264 (20130101); C04B
35/6303 (20130101); H05K 1/165 (20130101); C04B
2235/3279 (20130101); C04B 2235/3281 (20130101); C04B
2235/3284 (20130101); C04B 2235/3296 (20130101); C04B
2235/3298 (20130101); C04B 2235/3418 (20130101); C04B
2235/365 (20130101); C04B 2235/656 (20130101); C04B
2235/6562 (20130101); H01F 1/0027 (20130101); H01F
1/344 (20130101); H05K 1/095 (20130101); H05K
3/4664 (20130101); H05K 2201/086 (20130101) |
Current International
Class: |
H01F
17/04 (20060101); H01F 017/04 () |
Field of
Search: |
;336/177,200,232,233,219
;428/210 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
A Design of Experiment for a Tape Casting Process, Berry et al.,
Proc. 2000 International Symposium on Microelectronics, Boston, MA
Sep. 20-22, pp. 150-155. .
Lead-Free Multilayer Dielectric System for Telecommunications,
Wahlers, et al., 103.sup.rd Annual Meeting and Exposition, The
American Ceramic Society, Indianapolis, IN, Apr. 22-25, 2001, (oral
presentation, no published version). .
Lead Free Dielectric Tape System for High Frequency Applications,
Feingold, et al., Proc. 2001 International Symposium on
Microelectronics, Baltimore, MD, Oct. 9-11, pp. 133-137. .
Low Profile Transformers Using Low Temperature Co-Fire Magnetic
Tape, Bielawski, et al., Telecom Hardware Solutions 2002, Plano,
TX, May 15-16 (oral presentation, no published version). .
Capacitor and Inductor Compositions for Buried Components,
Feingold, et al., IMAPS--The International Microelectronics and
Packaging Society, Advanced Technology Workshop on Passive
Integration, Ogunquit, ME, Jun. 19-21, 2002, (oral presentation, no
published version). .
Compliant Dielectric and Magnetic Materials for Buried Components,
Feingold, et al., IMAPS. Keystone 2002 Symposium, Bethlehem, PA,
Jun.6, 2002 (oral presentation, no published version). .
Low Profile LTCC Transformers, Wahlers, et al., Proc. 2002
International Symposium on Microelectronics, Denver, CO, Sep. 4-6,
pp. 76-80..
|
Primary Examiner: Lam; Cathy F.
Attorney, Agent or Firm: Weingarten, Schurgin, Gagnebin
& Lebovici LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority from U.S. Provisional Patent
Application Ser. No. 60/337,289, filed Nov. 8, 2001, which is
incorporated in its entirety herein.
Claims
What is claimed is:
1. A materials system for manufacturing multilayer magnetic
components comprising: (a) a magnetically permeable ferrite
material selected from the group consisting of Low Temperature
Cofire Ceramic (LTCC) ferrite tape and thick film ferrite paste,
(b) metal conductor paste, wherein said metal conductor paste is
disposed to be in contact with said ferrite material, and (c) thick
film dielectric paste, wherein said film dielectric paste is
disposed to be in contact with and cover said metal conductor, and
wherein said ferrite material has a flux containing approximately
0.2-5 weight % Bi.sub.2 O.sub.3, and further wherein said metal
conductor paste comprises a metal selected from the group
consisting of silver, gold, copper, palladium, platinum and alloys
thereof.
2. The material system of claim 1 wherein said metal conductor
paste comprises: a. a thick film buried silver conductor paste, b.
a thick film via-fill silver conductor paste and c. a thick film
solderable top layer silver conductor paste.
3. The material system of claim 2 wherein said thick film buried
silver conductor paste has solids composition comprised by weight %
of 93 to 100% silver, 0 to 3% grain growth inhibitor and 0 to 4% of
a glass or binder.
4. The material system of claim 2 wherein said thick film via-fill
silver conductor paste has solids composition comprised by weight %
of 94 to 100% silver and 0 to 6% binder.
5. The material system of claim 2 wherein said thick film
solderable top layer silver conductor paste has solids composition
comprised by weight % of 81 to 97% silver, 2 to 7% platinum and, 1
to 12% binder.
6. The material system of claim 1 wherein said ferrite has solids
composition in percent by weight comprising 84.8 to 98.8%
Ni--Cu--Zn ferrite, 0.2 to 5% Bi.sub.2 O.sub.3, 0 to 5% PbO, 0 to
15% filler and 0 to 10% glass binder.
7. The material system of claim 6 wherein said ferrite comprises by
weight % approximately 5 to 10% Ni, 1 to 5% Cu, 10 to 20% Zn, and
0.1 to 5% PbO.
8. The material system of claim 1 wherein said ferrite comprises
approximately 1 to 5 weight % Bi.sub.2 O.sub.3.
9. The material system of claim 8 wherein said ferrite comprises
approximately 1.25 to 3 weight % Bi.sub.2 O.sub.3.
10. The material system of claim 1 wherein said thick film
dielectric paste has solids composition comprised by weight % of 35
to 65% borosilicate glass and 35 to 6% filler.
11. The material system of claim 10 wherein said borosilicate glass
has thermal expansion coefficient in the range of 8 to 11
ppm/.degree. C. and softening point less than 700.degree. C.
12. The material system of claim 1 which further comprises a grain
growth inhibitor.
13. The material system of claim 1 wherein the thickness of said
dielectric paste is between 20 and 70 microns.
14. The material system of claim 1 wherein said magnetic components
have magnetic coupling coefficient greater than 0.95 and dielectric
breakdown voltage greater than 500V/mil between elements.
15. The material system of claim 1 further comprising at least one
registration hole formed in said ferrite material.
16. The material system of claim 1 further comprising at least one
via hole formed in said ferrite material.
17. The material system of claim 16 wherein the at least one via
hole is operable to provide for an electrical interconnect.
Description
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
N/A
BACKGROUND OF THE INVENTION
Traditional miniature wire-wound magnetic circuit components are
relatively simple in design compared to the miniature multilayer
magnetic components manufactured using the materials system of this
invention. Unfortunately this simple design of the traditional
miniature wire-wound magnetic components is not conducive towards
current trend of increased functionality and integration in
electronic design and manufacturing of the circuit boards so as to
reduce cost, size and, weight of the device. In the traditional
miniature wire-wound magnetic component a very tiny wire is wound
around a magnetic core to form the induction coil. Automation of
the coil winding process is not feasible for such very small sized
components. Hence manufacturing of such very small sized components
is costly due to the use of a labor-intensive manual wire-winding
process. By design, these miniature magnetic circuit components
have following two shortcomings: (1) too large a form factor
(height in relation to base of a component mounted on the circuit
board) compared to that of the chip scale components needed for
size and weight reduction of electronic devices of today and
tomorrow and (2) cannot be integrated into the typical multilayer
microelectronic manufacturing process to design a
multi-functional/increased functionality component with inductive
as well as capacitive and/or resistive functions for potential
lowering of cost, size and weight of the electronic device. This
approach to potential cost reduction by increasing the
functionality of a device is well known to those knowledgeable in
the art of hybrid microelectronics and semiconductor devices and
packages. Part of this trend is being accomplished by an ongoing
increased level of integration on the IC (Integrated Circuit) chips
by the semiconductor manufacturers. Even so a large number of
passive components are needed to interconnect and support these
ICs. Thus, there is an ongoing effort to miniaturize these passive
components so as to reduce the size and weight of the circuit
board. Miniaturization of resistors and capacitors has been
accomplished by utilizing techniques such as discrete surface
mounted chip components and thick film buried components.
Miniaturization of the conventional wire-wound magnetic components
such as inductors and transformers has been conceptually
demonstrated by replacing the wire-wound induction coil with a
multilayer microelectronic design that uses a thick film conductor
deposited on laminated layers of a ferromagnetic ceramic to form
the induction coil of the magnetic component. These multilayer
magnetic components are manufactured in accordance with processing
steps typical of LTCC and thick film technologies. Compared to the
conventional wire-wound miniature magnetic component the multilayer
component design has following advantages: (1) magnetic component
with a chip scale form factor for potential reduction in circuit
size and weight, (2) potential to lower manufacturing cost by
process automation and, (3) potential for additional lowering of
device cost with increased functionality by integrating inductive
as well as capacitive and/or resistive functions on a
component.
U.S. Pat. Nos. 5,312,674; 5,349,743 and 6,054,914 disclose designs
and methods for manufacturing non wire-wound, monolithic,
multilayer transformers (magnetic circuit components) using Low
Temperature Cofired Ceramic (LTCC) technology and High Temperature
Cofired Ceramic Technology. A thick film paste or ceramic green
tape of a ferromagnetic material is used to form a single magnetic
layer. Each layer acts as a substrate for the next layer in the
sequential build-up of the multilayer magnetic component.
Pluralities of such magnetic layers are laminated on top of each
other to form the multilayer transformer. When using thick film
paste of the ferromagnetic material, the multilayer magnetic
component is manufactured by laminating individual layers of a
dielectric ceramic green tape screen-printed with the ferromagnetic
paste. Thick film conductors are screen-printed on individual
magnetic layers to form a part of the electrical winding of the
transformer. The primary and the secondary windings can be placed
on the same magnetic layer or spread vertically over several
magnetic layers through the multilayer component. When such
windings extend over more than one magnetic layer vias or holes are
provided at appropriate locations through the magnetic layers to
facilitate interconnection between the windings on different
magnetic layers. These vias are filled with the thick film
conductor to complete the electrical interconnection. In accordance
with the typical multilayer component design and manufacturing
procedure each layer is independently punched with v as and
screen-printed with appropriate thick film pastes as needed. Then
all of these layers are laminated in appropriate sequence to form a
green multilayer component package that is fired into an integral
structure at an appropriate high temperature.
U.S. Pat. No. 6,198,374 discloses use of a lower permeability
dielectric on Nickel-Zinc-ferrite (Ni--Zn ferrite) layers to
improve the magnetic coupling coefficient of a multilayer
transformer and to improve the dielectric breakdown voltage between
the adjacent conductor layers in the multilayer transformer.
Without the use of the lower permeability dielectric, the
transformer design shown in U.S. Pat. No. 6,198,374 has uniform
magnetic permeability throughout the multilayer structure with
calculated theoretical magnetic coupling coefficient of 0.83 and a
breakdown voltage of 2400VAC with a 7-mil thick ferrite layer
(345VAC/mil for a 7-mil thick ferrite layer). By applying the low
permeability dielectric over the thick film conductor in specific
areas the magnetic coupling coefficient of such a transformer is
improved to approximately 0.95 with improved dielectric breakdown
voltage.
The concept of non wire-wound magnetic components dates back over a
decade as shown in U.S. Pat. Nos. such as U.S. Pat. Nos. 3,833,872
and 4,547,961. During these past few years have appeared miniature
multilayer non wire-wound transformers. These multilayer
transformers of earlier inventions used materials systems that were
not necessarily optimized for such applications. As a result
multilayer transformers manufactured with these existing materials
systems had shortcomings in at least one of the following typical
application requirements for a miniature magnetic circuit
component: (1) Magnetic coupling coefficient less than 0.95, (2)
Dielectric breakdown voltage less than 1500V, a typical value for a
miniature wire-wound transformer (3) Form factor larger than
chip-scale, i.e. thickness of the transformer in relationship to
its area for mounting on a circuit board (4) Commercially proven
multilayer magnetic component design and, (5) Higher or comparable
cost compared to typical miniature wire-wound transformers. These
shortcomings have contributed to the slow pace of commercialization
of this multilayer technique towards miniaturization and
replacement of conventional wire-wound magnetic components such as
inductors and transformers.
Tape casting, also known as doctor blade casting or knife casting
is a well known technique used for casting thin, flat, sheets of a
ceramic material using a slip of the said material. The tape
casting slip consists of organic and inorganic components. In
accordance with their function in a tape casting slip these
components can be classified as follows: (1) The primary ceramic
powder, (2) Fluxes and sintering aids to assist in densification of
the fired ceramic, (3) Fillers or additives to adjust application
specific properties of the fired ceramic, (4) Resin to bond all the
inorganic particulates together in the green or dry, unfired state,
(to form a green tape), (5) Plasticizers to modify the properties
of the resin so as to make the ceramic green tape flexible for
forming or shaping and laminating, (6) Solvents as a medium to
dissolve the resins and plasticizers and suspend the inorganic
particulates to form the slip and, (7) Surfactants to facilitate
homogeneous dispersion of the particulates in the slip. In a basic
tape casting process a doctor blade is used to uniformly spread the
slip over a moving carrier film made of materials such as silicone
coated polyester. A ceramic green tape is obtained by evaporating
the solvent from the wet film. The left over resin and plasticizer
hold the ceramic particulates together while providing sufficient
flexibility to the ceramic green tape for subsequent forming
operations such as cutting to desired size and shape, drilling
holes and, lamination of multiple layers of the ceramic green tape
to form a multilayer ceramic green tape package (green package).
After all of the forming operations are complete the green package
is processed at an appropriate high temperature using a material
specific time-temperature firing protocol (firing profile) to
burn-off the remaining organic components and sinter and densify
the remaining inorganic components to form the fired multilayer
ceramic package (fired package). During this entire process,
beginning with making of the slip to cooling of the fired package
to ambient, all materials and processing related variable factors
can potentially influence the properties of the resultant fired
package. With reference to one specific application Berry et al.
("A Design of Experiment for a Tape Casting Process", C. Berry et.
al, Proc. 2000 International Symposium on Microelectronics,
pp.150-155) identified over forty possible variable factors in the
tape casting process and by utilizing the design of experiments
technique concluded that only six of these factors were critical
for manufacturing LTCC tapes with acceptable consistency and
repeatability for their application.
A typical thick film ink or paste for screen-printing contains, at
minimum, particulates of an inorganic active ingredient suspended
in an organic screening agent also known as vehicle. A typical
screening agent is a solution of a high molecular weight polymer
dissolved in a high boiling point alcohol. The ink is processed in
accordance with the typical print and fire thick film technique to
obtain a fired film of the active inorganic ingredient on a
substrate. The sintered active ingredient dominates the electrical
properties of the fired film. In practice the performance of the
fired film is also affected by materials related variables such as
shape and size of the particulates, chemical reactivity and
concentration of the solid phases in the film. Usually thick film
pastes are classified in accordance with their intended application
and as such the dominant active ingredient has electrical
properties typical of that general class of materials.
Following flow chart illustrates the typical procedural steps
generally carried out in LTCC processing of a microelectronic
multilayer magnetic component.
Typical LTCC process flow chart Cast LTCC ferrite tape .dwnarw. Cut
tape to size .dwnarw. Punch registration and via holes .dwnarw.
Fill vias with conductor paste and dry .dwnarw. Screen-print and
dry buried conductors .dwnarw. Screen-print and dry dielectric
layers .dwnarw. Screen-print and dry top/solderable conductor
.dwnarw. Laminate .dwnarw. Cut to size .dwnarw. Fire
When using a thick film ferrite paste to fabricate the multilayer
magnetic component the same general procedure is followed except a
substrate is used as a base for the multilayer component and the
thick film ferrite and conductors are deposited layer-by-layer per
the part design. The layers may be separately fired or cofired. In
the separate firing process each layer is deposited and fired and
forms the basis for the next layer in the part build up.
BRIEF SUMMARY OF THE INVENTION
This invention relates to a materials system comprised of following
five mutually compatible components,
(1) Low Temperature Cofire Ceramic (LTCC) ferrite tape or thick
film ferrite paste,
(2) Thick film buried silver conductor paste,
(3) Thick film via-fill silver conductor paste,
(4) Thick film solderable top layer silver conductor paste and,
(5) Thick film dielectric paste
which can be processed using typical LTCC and thick film techniques
at peak temperatures below 950.degree. C. to fabricate defect free,
flat multilayer magnetic components with magnetic coupling
coefficient greater than 0.95 and dielectric breakdown voltage
between adjacent elements greater than 500V/mil.
A Nickel-Copper-Zinc (Ni--Cu--Zn) ferrite powder is the material of
preference for this invention because it can be fired in air and
densified at relatively lower peak firing temperatures in the range
of 800.degree. C. to 950.degree. C. The LTCC ferrite tape and the
thick film ferrite paste of this invention have magnetic
permeability in the range of 25 to 750.
The three thick film silver conductor pastes of this invention, the
buried silver conductor paste, the via-fill silver conductor paste
and, the solderable top layer silver conductor paste are compatible
with the LTCC ferrite tape and ferrite paste of this invention. The
buried silver conductor is used to form planar induction coils
buried or sandwiched between ferrite layers. The via-fill silver
conductor is used to interconnect two or more of the planar, buried
silver coils through the thickness of the ferrite layers. The top
solderable silver conductor is used to connect the multilayer
magnetic component to the circuit board.
The thick film dielectric of this invention is compatible with the
ferrite and the thick film conductors of this invention and can be
cofired with these materials. The magnetic permeability of this
dielectric is less than that of the ferrite used in this invention.
The dielectric is screen-printed as a protective film on top of the
buried silver conductor. The presence of this dielectric helps in
redirecting the magnetic flux around the buried silver induction
coil there by enhancing the magnetic coupling coefficient for the
miniature multilayer magnetic component. The dielectric film also
acts as an insulator between adjacent buried silver conductor lines
there by enhancing the dielectric characteristics of interest in
applications of multilayer magnetic components.
In the preferred embodiment of this invention factors that enhance
the dielectric characteristics such as breakdown voltage,
insulation resistance and surge resistance are: use of Bi.sub.2
O.sub.3 as a sintering aid in the ferrite, higher peak firing
temperature of the multilayer package, reduced concentration of
solids in the thick film buried silver conductor paste, presence of
a grain growth inhibitor in the thick film buried silver conductor
paste and, thickness of the dielectric film.
BRIEF DESCRIPTION OF THE FIGURES
FIG. 1A shows a typical cut piece of LTCC ferrite tape.
FIG. 1B shows a typical cut piece of LTCC ferrite tape with punched
vias.
FIG. 1C shows a typical cut piece of the LTCC ferrite tape with
screen-printed thick film silver conductor.
FIG. 1D a typical cut piece of the LTCC ferrite tape with the
screen-printed thick film dielectric covering the screen-printed
thick film silver conductor.
FIG. 2 shows micrographs (taken on a scanning electron microscope,
with all micrographs having the same magnification) showing that
the ferrite body with larger grain size has higher magnetic
permeability. The magnetic permeability of the figures (in
.mu..sub.m)are: A: 100; B: 178; C: 258; D: 378.
Table 1 shows that magnetic permeability at different peak firing
temperatures.
Table 2 shows BDV at different concentrations of Bi.sub.2
O.sub.3.
Table 3 shows various properties at different peak
temperatures.
Table 4 shows properties at relatively high and low concentrations
of solids in silver paste.
Table 5 shows properties in paste with and without grain growth
inhibitor.
Table 6 shows BDV at different film thicknesses of dielectric.
Table 7 shows insulation resistance at different thicknesses of
dielectric film.
DETAILED DESCRIPTION OF THE INVENTION
The materials system and processing conditions of this invention
overcome the shortcomings of the prior art, thereby facilitating
commercialization of miniature multilayer magnetic components.
Using the materials system and processing conditions of this
invention prototype miniature multilayer magnetic components with
magnetic coupling coefficients greater than 0.95, dielectric
breakdown voltages greater than 500V/mil and, low chip-scale form
factor have been fabricated. Furthermore, the use of proven, thick
film and LTCC techniques that can be automated and are not as
labor-intensive should lower the cost of manufacturing such
miniature multilayer magnetic components when compared to the
current, labor-intensive manual wire-winding process.
A. Typical Manufacturing of LTCC Tapes and Thick Film Pastes
The constituents of all Low Temperature Cofire Ceramic (LTCC) tapes
and thick film pastes can be classified into two broad chemical
categories: inorganic and organic. The final fired ceramic is a
product of the chemical reaction between the particulates of the
inorganic constituents, usually referred to as solids in a tape
casting slip, a LTCC tape or a thick film paste. The organic
components act as a delivery medium for the solids by facilitating
handling and deposition of the inorganic particulates in the
desired shape and layout of the circuit for the intended
microelectronic application.
A typical LTCC tape manufacturing process starts off by preparing a
homogeneous, stable tape casting slip, which is a suspension of the
inorganic solids in the organic medium. This is accomplished by
processing all of the ingredients in a ball mill. The organics are
comprised of solvents, resins, plasticizers and surfactants. The
inorganics are comprised of the primary or dominant phase, fluxes
or sintering aids, and fillers. In a tape casting slip the organics
facilitate uniform spreading and deposition of the slip over a
moving carrier film such as silicone-coated polyester. After
deposition, the film is dried by evaporating some of the solvent
and allowing the remainder of the organics to bind the solids
together to form a flexible, strong LTCC tape of inorganic solids
held together in an organic polymer matrix loosely adhered to the
carrier film. The rheology of the slip and the flexibility of the
LTCC tape are governed by the organics that are selected in
accordance with the intended application for the LTCC tape.
A typical thick film paste manufacturing process starts off by
dissolving the polymer in the solvent to form a homogeneous fluid
known as organic screening agent or vehicle. Next, by means of a
three-roll mill the inorganic solids are uniformly dispersed in the
organic screening agent to form a homogeneous suspension called
thick film paste. In thick film pastes the organics facilitate
screen-printing of the paste in a defined pattern on a substrate.
After screen-printing the paste deposit is dried by evaporating
some of the solvent and allowing the remainder of the organics to
bind the solids together as a film adhered to the substrate. The
rheology of the paste is governed by the organics that are selected
in accordance with the intended application for the paste.
The rheology or viscosity of the tape casting slip and the thick
film paste is measured using a viscometer.
B. LTCC Ferrite Tape
The primary constituent of a LTCC ferrite tape is a ferrite powder
chosen in accordance with the intended application. The LTCC
ferrite tape of this invention is compatible with and can be
cofired with other embodiments of this invention: the thick film
ferrite paste, the thick film buried silver conductor paste, the
thick film via-fill silver conductor paste, the thick film
solderable top layer conductor paste and, the thick film dielectric
paste. In the preferred embodiment of this invention the LTCC
ferrite tape can be fired in air below 950.degree. C.
This invention relates in part to Nickel-Zinc (Ni--Zn) LTCC ferrite
tape with magnetic permeability in the range 20 to 750. Additional
components (e.g., Cu; see below) can be incorporated in the ferrite
tape. A Nickel-Copper-Zinc (Ni--Cu--Zn) ferrite powder is the
material of preference for this invention because it can be fired
in air and densified at relatively lower peak firing temperatures
in the range 800.degree. C. to 950.degree. C. Additives such as
fluxes or sintering aids used to promote densification of the
Ni--Cu--Zn ferrite include Bi.sub.2 O.sub.3, B.sub.2 O.sub.3, PbO,
CuO, V.sub.2 O.sub.5, ZnO and low melting glasses. The preferred
flux of this invention is Bi.sub.2 O.sub.3. Additives such as CaO,
Nb.sub.2 O.sub.5 and, SiO.sub.2 are used as fillers to modify the
electrical and magnetic properties of the ferrite so as to satisfy
the application requirements. In the preferred embodiment of this
invention the concentration of such additives is kept below 15%.
From the commercially available wide selection of resins,
plasticizers, solvents and, surfactants, those materials that are
compatible with each other and with the ferrite, fluxes and,
additives being used are selected for making the ferrite tape
casting slip of this invention. The resins used include polyvinyl
butyral and acrylic polymers. The plasticizers used include butyl
benzyl phthalate and polyethylene glycols. High boiling point
alcohols and aromatic hydrocarbons are used as solvents. The
surfactants of choice are fish oils and phosphates. Materials
related variables critical to the application and performance of
the LTCC ferrite tape are: (1) Size of the ferrite particulates in
the slip, (2) Particulate size and concentration of the flux, (3)
Particulate size and concentration of the filler, (4) Thickness of
the LTCC ferrite tape and, (5) Peak firing temperature for the
multilayer package. In the preferred embodiment of this invention
these critical variables have been optimized so as to enhance the
following performance characteristics of the miniature magnetic
components: (1) Magnetic coupling coefficient (2) Insulation
resistance, (3) Surge resistance and, (4) Dielectric breakdown
voltage. The LTCC ferrite tapes of this invention can be fired in
air below 950.degree. C. to manufacture flat miniature multilayer
magnetic components with magnetic coupling coefficient greater than
0.95 and dielectric breakdown voltage greater than 500V/mil.
In the preferred embodiment of this invention the LTCC ferrite tape
is comprised of (all concentrations being approximate and expressed
in percent weight):
(a) 52 to 75% Ni--Cu--Zn ferrite powder composed of 5 to 10% Ni, 1
to 5% Cu, 10-20% (preferably 15 to 20%) Zn, 40 to 50% Fe and
balance O, (the total solids level being 84-99% (preferably
90-98%), with magnetic permeability in the range of 25 to 750 and
average particle size in the range of 0.2 to 5 microns, preferably
0.5 to 2 microns,
(b) approximately 0.2 to 5% (preferably approximately 1-5% and more
preferably approximately 1.25-3%) Bi.sub.2 O.sub.3 as a flux or
sintering aid with an average particle size in the range of 0.2 to
5 microns, preferably 0.5 to 2 microns,
(c) 0 to 5% (preferably 0.1-5%) PbO as a flux or sintering aid with
an average particle size in the range of 0.2 to 5 microns,
preferably 0.5 to 2 microns
(d) 0 to 3% SiO.sub.2 as filler, with an average particle size in
the range of 0.2 to 5 microns, preferably 0.2 to 2 microns,
(e) 2 to 10% resin, (e.g. poly vinyl butyral)
(f) 1 to 5% plasticizer, (e.g. poly ethylene glycol)
(g) 15 to 40 % solvent (e.g. a mixture of methyl ethyl ketone and
ethanol (60:40) and,
(h) 0 to 1% surfactant (e.g. Menhaden fish oil or phosphate
ester.
(Note: examples of ingredients are generally mentioned only the
first time a category of component is discussed herein. These
ingredients are also suitable examples when the component is later
discussed.)
It will be apparent to those knowledgeable in the art that other
ferrites, fluxes and fillers can also be used in place of the
preferred materials of this invention, for example Mn--Zn ferrite,
fluxes such as B.sub.2 O.sub.3, PbO, CuO, V.sub.2 O.sub.5, ZnO and
low melting glasses with softening point less than 700.degree. C.
and, fillers such as CaO and Nb.sub.2 O.sub.5.
Sintering aids and fluxes like Bi.sub.2 O.sub.3, B.sub.2 O.sub.3,
PbO, CuO, V.sub.2 O.sub.5, ZnO, low melting glasses, etc. can be
added to reduce the firing temperature. Excessive amounts reduce
the permeability so additive concentrations are usually kept under
15%. Most preferred levels of Bi.sub.2 O.sub.3 range from 1.25 to
2.5%.
Typical ceramic particle size reduction techniques are used to
obtain finely divided powders of the inorganic solids with uniform
particle size distributions.
In the preferred embodiment of this invention, room temperature
(23.degree. C. to 27.degree. C.) viscosity of the ferrite slip for
tape casting is in the range of 1,000 to 4,000 cP, preferably 1,800
to 2,200 cP, at 10 RPM using a Number 4 disc spindle on Brookfield
RVT viscometer.
The ferrite slip is deposited using a doctor blade on a moving
carrier film and dried to form the LTCC ferrite tape. A carrier
film such as silicone-coated polyester facilitates removal of the
LTCC ferrite tape for subsequent typical LTCC manufacturing steps
for multilayer microelectronic components. Several slip and casting
process related variable parameters such as concentration of solids
in the slip, rheology of the slip, height of the doctor blade and,
speed of the carrier film determine the quality or physical
characteristics of the LTCC ferrite tape such as average thickness
of the tape and the uniformity of this average thickness across
entire cast length of the tape. In the preferred embodiment of this
invention the average thickness of the LTCC ferrite tape is 1 to 12
mils (25 to 300 microns) . The desired average thickness of the
tape and its uniformity depends on its intended application.
C. Thick Film Ferrite Paste
The primary constituent of a thick film ferrite paste is a ferrite
powder chosen in accordance with the intended application. The
thick film ferrite paste of this invention is compatible with and
can be cofired with other embodiments of this invention: the LTCC
ferrite tape, the thick film buried silver conductor paste, the
thick film via-fill silver conductor paste, the thick film
solderable top layer silver conductor paste and, the thick film
dielectric paste. In the preferred embodiment of this invention the
thick film ferrite paste can be fired in air below 950.degree.
C.
This invention relates in part to Nickel-Zinc (Ni--Zn) thick film
ferrite paste with magnetic permeability in the range 20 to 750.
Additional components (e.g., Cu; see below) can be incorporated in
the ferrite paste. The Nickel-Copper-Zinc (Ni--Cu--Zn) ferrite
powder is the material of preference for this invention because it
can be fired in air and densified at relatively lower peak firing
temperatures in the range 800.degree. C. to 980.degree. C.
Additives such as fluxes or sintering aids used to promote
densification of the Ni--Cu--Zn ferrite include Bi.sub.2 O.sub.3,
B.sub.2 O.sub.3, PbO, CuO, V.sub.2 O.sub.5, ZnO and low melting
glasses. The preferred flux of this invention is Bi.sub.2 O.sub.3.
Additives such as CaO, Nb.sub.2 O.sub.5 and, SiO.sub.2 are used as
fillers to modify the electrical and magnetic properties of the
ferrite so as to satisfy the application requirements. In the
preferred embodiment of this invention the concentration of such
additives is kept below 20%. The thick film ferrite paste consists
of particulates of the ferrite, flux and filler suspended in a
screening agent. The screening agent of choice is a solution of
ethyl cellulose dissolved in a high boiling point alcohol.
Materials related variables critical to the application and
performance of the thick film ferrite paste are: (1) Size of the
ferrite particulates, (2) Particulate size and concentration of the
flux, (3) Particulate size and concentration of the filler, (4)
Concentration of the solids in the paste and, (5) Peak firing
temperature for the ferrite. In the preferred embodiment of this
invention these critical variables have been optimized so as to
enhance the following performance characteristics of the miniature
magnetic components: (1) Magnetic permeability (2) Insulation
resistance, (3) Surge resistance and, (4) Dielectric breakdown
voltage. The thick film ferrite pastes of this invention can be
fired in air below 950.degree. C. to manufacture miniature
multilayer magnetic components with magnetic coupling coefficient
greater than 0.95 and dielectric breakdown voltage greater than
500V/mil.
In the embodiment of this invention the thick film ferrite paste is
essentially similar to the LTCC tape, but is preferentially
comprised of:
(a) 62 to 80% of Ni--Cu--Zn ferrite powder composed of 5 to 10% Ni,
1 to 5% Cu, 15 to 20% Zn, 40 to 50% Fe and balance O, with magnetic
permeability in the range of 25 to 750 and average particle size in
the range of 0.2 to 5 microns, preferably 0.5 to 2 microns,
(b) approximately 0.2 to 5% (preferably approximately 1-5% and more
preferably approximately 1.25-3%) Bi.sub.2 O.sub.3 as a flux or
sintering aid with an average particle size in the range of 0.2 to
5 microns, preferably 0.5 to 2 microns,
(c) 0.1 to 5% PbO as a flux or sintering aid with an average
particle size in the range of 0.2 to 5 microns, preferably 0.5 to 2
microns
(d) 0 to 10% glass as a binder with an average particle size in the
range of 0.2 to 5 microns, preferably 0.5 to 2 microns
(e) 0 to 10% SiO.sub.2 as filler, with an average particle size in
the range of 0.2 to 5 microns, preferably 0.2 to 2 microns,
(f) 1 to 5% ethyl cellulose as resin and,
(g) 14 to 35% high boiling point alcohol (e.g. TEXANOL (a trademark
of Kodak) or 2,2,4-Trimethyl-1,3-pentanediol Monoisobutyrate) as
solvent and,
It will be apparent to those knowledgeable in the art that other
ferrites, fluxes and fillers can also be used in place of the
preferred materials of this invention, for example Mn--Zn ferrite,
fluxes such as B.sub.2 O.sub.3, CuO, V.sub.2 O.sub.5, ZnO and low
melting glasses with softening point less than 700.degree. C. and,
fillers such as CaO and Nb.sub.2 O.sub.5.
Typical ceramic particle size reduction techniques are used to
obtain finely divided powders of the inorganic solids with uniform
particle size distributions.
In the preferred embodiment of this invention, room temperature
(23.degree. C. to 27.degree. C.) viscosity of the thick film
ferrite paste is in the range of 150 to 300 kcP at a shear rate of
1.05/sec, using a cylindrical spindle on Brookfield RVT
viscometer.
The thick film ferrite paste is deposited in the desired layout or
pattern on a substrate by the screen-printing technique. Several
paste and screen-printing related variable parameters such as
concentration of solids in the paste, rheology of the paste, mesh
count of the screen and thickness of the screen determine the
quality or physical characteristics of the screen-printed ferrite
film or print such as thickness and sharpness of the edges of the
print.
D. Thick Film Buried Silver Conductor Paste
A silver powder is the primary ingredient in a thick film silver
conductor paste. The thick film buried silver conductor paste of
this invention is compatible with and can be cofired with other
embodiments of this invention: the LTCC ferrite tape, the thick
film ferrite paste, the thick film dielectric paste, the thick film
via-fill silver conductor paste and, the thick film solderable top
layer conductor paste.
The thick film buried silver conductor paste is screen-printed on
the ferrite layer to form a planar induction coil. Pluralities of
such layers are laminated to form the multilayer magnetic
component. The conductor paste is comprised of particulates of
silver and a ceramic binder suspended in the screening agent. The
screening agent of choice is a solution of ethyl cellulose
dissolved in a high boiling point alcohol. In the preferred
embodiment of this invention, the thick film buried silver
conductor paste consists of silver particulates 1 .mu.m to 10
.mu.un in size. Materials related variables critical to the
application and performance of the buried silver thick film
conductor paste are: (1) Concentration of silver particulates, (2)
Size of silver particulates, (3) Concentration of ceramic binder,
(4) Size of ceramic binder particulates and, (5) Chemical
compatibility between the ceramic binder and the ferrite. In the
preferred embodiment of this invention these critical variables
have been optimized so as to enhance the following performance
characteristics of the miniature magnetic components: (1) Flatness
or warpage of the miniature component, (2) Insulation resistance,
(3) Surge resistance and, (4) Dielectric breakdown voltage. The
thick film buried silver conductor paste of this invention can be
fired in air below 950.degree. C. (preferably about 850-950C.) to
manufacture flat, miniature multilayer magnetic components with
magnetic coupling coefficient greater than 0.95 and dielectric
breakdown voltage greater than 500V/mil.
In the preferred embodiment of this invention the thick film buried
silver conductor paste is comprised of:
(a) 65 to 85% silver powder with an average particle size in the
range of 1 to 10 microns, preferably 4 to 5 microns,
(b) 0 to 3% ceramic binder (e.g. copper oxide, cadmium oxide or
glasses) with an average particle size in the range of 1 to 10
microns, preferably 1 to 2 microns,
(c) 0 to 2% grain growth inhibitor,
(d) 1 to 5% ethyl cellulose as resin and,
(e) 14 to 35% high boiling point alcohol as solvent.
In the preferred embodiment of this invention room temperature
(23.degree. C. to 27.degree. C.) viscosity of the thick film buried
silver conductor paste is in the range of 100 to 300 kcP at a shear
rate of 1.05/sec, using a cylindrical spindle on Brookfield RVT
viscometer.
The thick film buried silver conductor paste is deposited in the
desired layout or pattern by the screen-printing technique on the
LTCC ferrite tape or the screen-printed and dried ferrite paste
deposit. Several paste and screen-printing related variable
parameters such as concentration of solids in the paste, rheology
of the paste, mesh count of the screen and thickness of the screen
determine the quality or physical characteristics of the
screen-printed silver conductor film or print such as thickness and
sharpness of the edges of the print.
The thick film buried silver conductor paste forms the planar
induction coil sandwiched between the ferrite layers of the
miniature multilayer magnetic component. In the preferred
embodiment of this invention design specific areas of the buried
silver conductor may be covered with the thick film dielectric
paste of this invention.
While silver is the preferred conductor in applications where
conductivity and cost are the key requirements, Au, Cu, Pt, Pd and
alloys of these can be used and are preferred in other
applications, ones in which firing must be higher than 960 C. (the
melting point of silver). Copper is a good choice for systems which
can be fired in inert atmospheres like nitrogen. It, like silver,
is inexpensive and has a low value of electrical resistance. In
addition, it has a higher melting point than silver.
E. Thick Film via-fill Silver Conductor Paste
A silver powder is the primary ingredient in a thick film silver
conductor paste. The thick film via-fill silver conductor paste of
this invention is compatible with and can be cofired with other
embodiments of this invention: the LTCC ferrite tape, the thick
film ferrite paste, the thick film buried silver conductor paste,
the thick film solderable top layer silver conductor paste and, the
thick film dielectric paste.
The thick film via-fill silver conductor is used to interconnect
the different layers of planar thick film buried silver conductor
in the multilayer package. The thick film via-fill silver conductor
paste is comprised of particulates of silver and a ceramic binder
suspended in a screening agent. The screening agent of choice is a
solution of ethyl cellulose dissolved in a high boiling point
alcohol. In the preferred embodiment of this invention, the thick
film via-fill silver conductor paste consists of silver
particulates 2 .mu.m to 12 .mu.m in size, ceramic binder with
particulate size comparable to that of silver and, the screening
agent. Materials related variables critical to the application and
performance of the thick film via-fill silver conductor paste are:
(1) Concentration of silver, (2) Size of silver particulates, (3)
Concentration of ceramic binder, (4) Size of ceramic binder
particulates, (5) Thermal expansion coefficient of the ceramic
binder and, (6) Chemical compatibility between the ceramic binder
and the ferrite. In the preferred embodiment of this invention
these critical variables have been optimized so as to form reliable
electrical interconnects between the buried planar induction coils
screen-printed on the ferrite layers of the multilayer magnetic
component. The thick film via-fill silver conductor paste of this
invention can be fired in air below 950.degree. C. to manufacture
flat miniature multilayer magnetic components with magnetic
coupling coefficient greater than 0.95 and dielectric breakdown
voltage greater than 500V/mil.
In the preferred embodiment of this invention the thick film
via-fill silver conductor paste is comprised of:
(a) 63 to 90% silver powder with an average particle size in the
range of 1 to 10 microns, preferably 4 to 5 microns,
(b) 0 to 5% ceramic binder with an average particle size in the
range of 1 to 10 microns, preferably 1 to 2 microns,
(c) 1 to 5% ethyl cellulose as resin and,
(d) 5 to 35% high boiling point alcohol as solvent.
In the preferred embodiment of this invention, room temperature
(23.degree. C. to 27.degree. C.) viscosity of the thick film
via-fill silver conductor paste is in the range of 1,000 to 2,000
kcP at a shear rate of 0.1/sec, using a cylindrical spindle on
Brookfield RVT viscometer.
The thick film via-fill silver conductor paste is deposited in the
vias (or feed-through holes) present in the LTCC ferrite tape or
the screen-printed and dried ferrite paste deposit. Several paste
and screen-printing related variable parameters such as
concentration of solids in the paste, rheology of the paste, mesh
count of the screen and thickness of the screen determine the
volume of the paste incorporated into the via. The thick film
via-fill silver conductor paste interconnects the planar induction
coil formed by the thick film buried silver conductor sandwiched
between the ferrite layers of the miniature multilayer magnetic
component.
F. Thick Film Solderable Top Layer Conductor Paste
A silver powder is the primary ingredient in a thick film silver
conductor-paste. The thick film solderable silver conductor paste
of this invention is compatible with and can be cofired with other
embodiments of this invention: the LTCC ferrite tape, the thick
film ferrite paste, the thick film buried silver conductor paste,
the thick film via-fill conductor paste and, the thick film
dielectric paste.
The thick film solderable top layer silver conductor paste of this
invention is designed for solder pad applications on the surface of
the miniature multilayer magnetic component. This conductor can be
either cofired or separately fired on the ferrite with excellent
solderability and leach resistance. The conductor paste is
comprised of particulates of precious metal and ceramic binders
suspended in a screening agent. The screening agent of choice is a
solution of ethyl cellulose dissolved in a high boiling point
alcohol. In the preferred embodiment of this invention, the thick
film solderable top layer silver conductor paste consists of silver
particulates 1 .mu.m to 4 .mu.m in size, platinum particulates less
than 1 .mu.m in size, a ceramic binder and, the screening agent.
Materials related variables critical to the application and
performance of the thick film solderable top layer silver conductor
paste are: (1) Concentration of silver, (2) Size of silver
particulates, (3) Concentration of platinum, (4) Size of platinum
particulates, (5) Concentration of ceramic binder, (6) Size of
ceramic binder particulates, (7) Thermal expansion coefficient of
the ceramic binder and, (8) Chemical compatibility between the
ceramic binder and the ferrite. In the preferred embodiment of this
invention these critical variables have been optimized so as to
enhance the following performance characteristics of the miniature
magnetic components: (1) Flatness or warpage of the miniature
component, (2) Insulation resistance, (3) Surge resistance, (4)
Dielectric breakdown voltage, (5) Magnetic coupling coefficient
and, (6) Quality factor. The thick film solderable top layer silver
conductor paste of this invention can be fired in air below
950.degree. C. to manufacture flat miniature multilayer magnetic
components with magnetic coupling coefficient greater than 0.95 and
dielectric breakdown voltage greater than 500V/mil.
In the preferred embodiment of this invention the thick film
solderable top layer silver conductor paste is comprised of:
(a) 55 to 70% silver powder with an average particle size in the
range of 1 to 4 (preferably 2 to 3) microns,
(b) 2 to 16% (preferably 2 to 6%) platinum powder with an average
particle size less than 4 microns, preferably less than 1
micron,
(c) 1 to 10% ceramic binder with an average particle size in the
range of 1 to 2 microns,
(c) 1 to 5% ethyl cellulose as resin and,
(d) 14 to 35% high boiling point alcohol as solvent.
Optionally 2 to 6% Bi.sub.2 O.sub.3 can be included.
In the preferred embodiment of this invention, room temperature
(23.degree. C. to 27.degree. C.) viscosity of the thick film
solderable top layer silver conductor paste is in the range of 200
to 400 kcP at a shear rate of 1.05/sec, using a cylindrical spindle
on Brookfield RVT viscometer.
The thick film solderable top layer silver conductor paste is
deposited in the desired layout or pattern by the screen-printing
technique on the LTCC ferrite tape or the screen-printed ferrite
paste deposit. Several paste and screen-printing related variable
parameters such as concentration of solids in the paste, rheology
of the paste, mesh count of the screen and thickness of the screen
determine the quality or physical characteristics of the
screen-printed silver conductor film or print such as thickness and
sharpness of the edges of the print. The thick film solderable top
layer silver conductor paste is used to form terminal pads
essential for soldering the miniature multilayer magnetic component
to other parts of the circuit board.
G. Thick Film Dielectric Paste
The primary components of a typical thick film dielectric paste are
glasses and fillers chosen in accordance with the intended
application. The thick film dielectric paste of this invention is
compatible with other embodiments of this invention: the LTCC
ferrite tape, the thick film ferrite paste, the thick film buried
silver conductor paste, the thick film via-fill silver conductor
paste and, the thick film solderable top layer conductor paste.
The thick film dielectric paste of this invention is designed to
improve the dielectric characteristics such as breakdown voltage,
insulation resistance and surge resistance between the adjacent
silver conductor traces buried in the multilayer magnetic
component. This dielectric also helps in enhancing the quality
factor of the multilayer magnetic component by redirecting the
magnetic flux around the thick film buried silver induction coil so
as to enhance its magnetic coupling coefficient. Redirection of
magnetic flux in the ferrite layers needs a dielectric with
magnetic permeability lower than that of the ferrite layers on
which the said dielectric is deposited. The dominant ingredients of
the dielectric paste are particulates of glasses and ceramic
fillers suspended in a screening agent. The screening agent of
choice is a solution of ethyl cellulose dissolved in a high boiling
point alcohol. In the preferred embodiment of this invention the
glasses of choice have thermal expansion coefficient in the range 8
to 11 ppm/.degree. C. and have a working temperature comparable to
the peak temperature used for firing the LTCC ferrite tape.
Materials related variables critical to the application and
performance of the thick film dielectric paste are: (1) Size of
glass particulates, (2) Thermal expansion coefficient of glass, (3)
Softening point of glass, (4) Size of filler particulates and, (5)
Thermal expansion coefficient of filler. In the preferred
embodiment of this invention these critical variables have been
optimized so as to enhance the following performance
characteristics of the miniature multilayer magnetic components:
(1) Flatness or warpage of the miniature component, (2) Insulation
resistance, (3) Surge resistance, (4) Dielectric breakdown voltage,
(5) Magnetic coupling coefficient and, (6) Quality factor. The
thick film dielectric paste of this invention can be fired in air
below 950.degree. C. to manufacture flat miniature multilayer
magnetic components with magnetic coupling coefficient greater than
0.95 and dielectric breakdown voltage greater than 500V/mil.
In the preferred embodiment of this invention the thick film
dielectric paste is comprised of:
(a) 22 to 55% borosilicate glass powder with an average particle
size in the range of 0.5 to 2 microns with softening point below
700.degree. C. and coefficient of thermal expansion in the range of
8 to 11 ppm/.degree. C., preferably 9 to 10 ppm/.degree. C.,
(b) 22 to 55% inorganic filler (e.g. alumina, stabilized zirconia
or zinc oxide) with an average particle size in the range of 0.5 to
2 microns and coefficient of thermal expansion in the range of 6 to
12 ppm/.degree. C.,
(c) 0 to 10% inorganic flux (e.g. bismuth trioxide (Bi.sub.2
O.sub.3), lead oxide (PbO), or low melting glasses (glasses with
melting point below about 600 C.)) with an average particle size
less than 10 microns,
(d) 1 to 5% ethyl cellulose as resin and,
(e) 14 to 35 % high boiling point alcohol as solvent.
In the preferred embodiment of this invention, room temperature
(23.degree. C. to 27.degree. C.) viscosity of the thick film
dielectric paste is in the range of 200 to 400 kcp at a shear rate
of 1.05/sec, using a cylindrical spindle on Brookfield RVT
viscometer.
The thick film dielectric paste is deposited in the desired layout
or pattern by the screen-printing technique on either of the
following inventions of this embodiment: the LTCC ferrite tape, the
screen-printed and dried thick film ferrite paste deposit or the
thick film buried silver conductor paste. Several paste and
screen-printing related variable parameters such as concentration
of solids in the paste, rheology of the paste, mesh count of the
screen and thickness of the screen determine the quality or
physical characteristics of the screen-printed dielectric film or
print such as thickness and sharpness of the edges of the
print.
In the preferred embodiment of this invention the thick film
dielectric has magnetic permeability lower than that of the ferrite
on which it is printed so as to redirect the magnetic flux through
the bulk of the ferrite core of the miniature, multilayer magnetic
component thereby enhancing its magnetic coupling coefficient to
0.95 or higher values (theoretical maximum being 1.0).
In the preferred embodiment of this invention the thick film
dielectric electrically insulates adjacent thick film buried silver
conductor lines thereby enhancing the dielectric breakdown voltage
of the miniature, multilayer magnetic component to values greater
than 500V/mil.
H. Sample Preparation, Test Procedures And Example
The dielectric and inductive characteristics of the materials
system of this invention are evaluated using multilayer test parts
fabricated in accordance with the typical LTCC and thick film
manufacturing techniques. Fabrication of all test parts can be
split into two distinct functions: (1) Preparation of the green
multilayer package and (2) Firing of the green multilayer
package.
Preparation of the green multilayer package is along following
typical processing steps. At the outset, as per the test part
design the LTCC ferrite tape is cut to size (FIG. 1A). If called
for in the test part design, via-holes are punched (FIG. 1B). Next
the thick film silver conductor paste is screen-printed on the
ferrite tape and dried in a laboratory oven at a temperature of
120.degree. C. to 130.degree. C. for duration of 5 to 10 minutes
(FIG. 1C) . The layout of the silver conductor film depends on the
characteristics to be evaluated. If called for in the test part
design, the vias are filled with the via-fill thick film conductor.
If called for in the test part design, the thick film dielectric
paste is screen-printed over the dry silver conductor film (FIG.
1D). The screen-printed dielectric film is dried in a laboratory
oven at a temperature of 120.degree. C. to 130.degree. C. for
duration of 5 to 10 minutes. After completion of the
screen-printing steps the parts are ready for lamination. In
accordance with the test part design a combination of blank ferrite
layers and screen-printed ferrite layers are stacked and laminated
to form the green, multilayer test package comprised of buried
silver conductor with and without the dielectric film. When
preparing multilayer packages with the materials system of this
invention the optimum laminating parameters are: 2 to 12 minutes at
1,000 to 2,000 PSI pressure and 65.degree. C. to 75.degree. C.
temperature. The laminated package is then fired using a specific
time-temperature firing protocol (firing profile) to form the
finished test part.
In high temperature manufacturing process of microelectronic
components using LTCC tapes and thick film pastes, the typical
time-temperature firing profile can be classified into three broad
processing phases: first, the heating phase from ambient to peak
firing temperature followed by the soaking phase at peak
temperature and lastly the cooling phase from peak temperature to
ambient. With the onset of the heating phase, the organic
constituents begin to burn-off leaving behind a porous compact of
particulates of the inorganic solids supported by the setter-plate.
This portion of the heating phase during which the organics
burn-off is classified as the burn-off stage. The time span needed
or allocated for burn-off stage depends on the amount of organics
present in the materials being fired and ranges from a few minutes
when firing just a single layer to several hours when firing a
multilayer package. The maximum temperature used. in the burn-off
stage depends on the type of organics used and may be as high as
650.degree. C. During the remainder of the heating phase and
continuing through the soaking phase at peak firing temperature the
inorganic particulates interact with each other leading to
sintering and densification of these constituents. These
constituents may also react chemically leading to formation of new
material phases that were not part of the original chemical
composition. The soak time and peak firing temperature used depend
on the physical and chemical properties of the inorganic
constituents and the desired microelectronic properties and
performance for intended application. With the onset of cooling
phase the interaction between the inorganic constituents begins to
slow down there by freezing the microstructure that imparts the
electrical properties and performance characteristics to the
finished or fired microelectronic component or test part.
The following examples are intended to further illustrate, not
limit, the invention.
Example 1
Preparing a Component
A non-wire wound, miniature, multilayer magnetic circuit component
was constructed using the starting materials and techniques
described herein. The component contained ferrite tape, thick film
buried silver conductor, thick film via-fill silver conductor,
thick film solderable top layer silver conductor and thick film
dielectric. Following table lists the sequence of processing steps
used and describes the layer-by-layer design of the multilayer
component. (In an actual device, there may be circuits on several
layers. The examples of devices disclosed below had circuits on
only one layer or on none of the layers, since they were used for
testing purposes only.)
Multilayer part for evaluating the effect of Multilayer part thick
film for testing dielectric on Toroid for dielectric the dielectric
testing properties of properties of Processing magnetic the ferrite
the ferrite steps permeability tape tape 1st step: cut 10 pieces,
10 pieces, each 10 pieces, each the ferrite each 1 in 1 in square 1
in square tape to size square 2nd step: not thick film thick film
screen print applicable buried silver buried silver and dry
conductor on 1 conductor on 1 conductor piece of piece of ferrite
tape ferrite tape 3rd step: not not applicable thick film screen
print applicable dielectric on and dry top of the dry dielectric
silver print 4th step: punch not punch vias in 5 punch vias in 5
vias applicable ferrite tape ferrite tape pieces pieces 5th step:
align all 10 align ferrite align ferrite laminate pieces tape
pieces tape pieces ferrite tape with vias on with vias on pieces at
1500 top of silver top of silver PSI for 5 min print and all print
and all at 70 C. other pieces other pieces below the below the
silver print silver print 6th step: punch punch to not applicable
not applicable to size form a ring Layer by layer design 1st layer
ferrite tape ferrite tape ferrite tape 2nd layer ferrite tape
ferrite tape ferrite tape 3rd layer ferrite tape ferrite tape
ferrite tape 4th layer ferrite tape ferrite tape ferrite tape 5th
layer ferrite tape ferrite tape ferrite tape 6th layer ferrite tape
thick film thick film buried silver buried silver conductor
conductor 7th layer ferrite tape ferrite tape thick film with vias
dielectric 8th layer ferrite tape ferrite tape ferrite tape with
vias with vias 9th layer ferrite tape ferrite tape ferrite tape
with vias with vias 10th layer ferrite tape ferrite tape ferrite
tape with vias with vias 11th layer not ferrite tape ferrite tape
applicable with vias with vias 12th layer not not applicable
ferrite tape applicable with vias
Example 2
Firing the Component
A green multilayer package or magnetic component fabricated from
the materials system of this invention can be fired using the
following typical firing profile. Starting at room temperature,
heat at a rate of 1.degree. C./min to 3.degree. C./min to the
burn-off step temperature in the range 400.degree. C. to
500.degree. C. Hold at this burn-off step temperature for a
duration of 15 minutes to 2 hours to facilitate optimum burn-off.
Then continue heating at a rate faster than 4.degree. C./min to the
peak firing temperature in the range of 850.degree. C. to
950.degree. C. Hold or soak at the peak firing temperature for
duration of 15 minutes to 4 hours depending on the desired
properties for the finished or fired multilayer package or magnetic
component. The preferred firing profile for the materials system of
this invention is as follows: Starting at room temperature, heat at
a rate of 2.degree. C./min to the burn-off step temperature,
450.degree. C. Hold at 450.degree. C. for a duration of 2 hours.
Then continue heating at a rate of 6.degree. C./min to the peak
firing temperature, 930.degree. C. Soak at the 930.degree. C. peak
firing temperature for duration of 3 hours.
It will be evident to those knowledgeable in the art that during
firing of LTCC multilayer packages the three important criteria in
the selection of the time-temperature protocol are: (1) Selection
of heating and cooling rates to allow fabrication of defect-free
parts in minimal time, (2) Selection of time-temperature parameters
for optimum burn-off and, (3) Selection of peak firing temperature
and soak time at peak temperature for fabricating parts with
desired optimum performance characteristics. Thus in the embodiment
of this invention the time-temperature parameters selected to
fabricated the miniature multilayer magnetic component are critical
only in so far as the peak firing temperature is below 960.degree.
C. (preferably 875-915C.) when using thick film silver conductors
with LTCC ferrite tapes.
Example 3
Testing Magnetic Permeability of Toroid
In this embodiment the magnetic permeability, .mu..sub.m is
measured using a toroid fabricated from a multilayer laminate of
the LTCC ferrite tape. Ten layers of the LTCC ferrite tape are
laminated and by means of a punch and a die set a green multilayer
ferrite-ring with an inside diameter of 0.25 inch and an outside
diameter of 0.625 inch is fabricated and fired to form the magnetic
core for the toroid test part. The three dimensions, inside
diameter, ID, outside diameter, OD and, thickness, t of the fired
ring are recorded. Next, ten loops of an insulted wire are wound
around the ring to form the toroid test part. An insulated wire, 26
AWG solid silver-plated OFHC copper 0.0055 KYNAR is used for the
toroid windings and to facilitate electrical connection to the test
equipment. The inductance, L of the toroid test part is measured
with Wayne Kerr (West Sussex, UK) Precision Magnetics Analyzer,
Model PMA 3260A, operating at 10 mA, 500 kHz. The magnetic
permeability, .mu..sub.m of the ferrite core is then given by the
formula:
the inductance L is in micro-Henry and the Geometric Constant, G is
a function of dimensions of the toroid and is defined by the
formula:
The magnetic permeability of the ferrite is influenced by the
chemical composition of the ferrite as well the grain size of the
ferrite characterized by the peak firing temperature used to fire
the LTCC ferrite tape multilayer test part. The effect of ferrite
chemistry and peak firing temperature on magnetic permeability is
evaluated by fabricating toroid test parts with five different LTCC
ferrite tape formulations, Tape A, Tape B, Tape C, Tape D and, Tape
E fired at four different peak firing temperatures. Data presented
in Table 1 shows that the magnetic permeability is higher at higher
peak firing temperature (Peak T). The micrographs (all at the same
magnification) presented in FIGS. 2A, 2B, 2C and, 2D show that the
multilayer ferrite body with larger ferrite grain size has higher
magnetic permeability, .mu..sub.m.
Example 4
Testing Dielectric Properties of the Component of the Materials
System of this Embodiment, the Dielectric
In the miniature multilayer magnetic component applications
characteristics of interest are: dielectric breakdown voltage,
insulation resistance and, surge resistance. These characteristics
are evaluated on a multilayer test part comprised of a thick film
inter-digitated conductor sandwiched between layers of LTCC ferrite
tape. Ten pieces, 1 inch wide by 1 inch long are cut from the LTCC
ferrite tape. On one of these pieces, the thick film buried silver
conductor paste is printed and dried. The screen-printed,
inter-digitated conductor layout has 15-mil wide, 15-mil space
silver lines, 169-squares long covering an area of 0.4 in.times.0.5
in. If called for in the test part design, the thick film
dielectric paste is screen-printed and dried to cover most of the
screen-printed and dried inter-digitated silver conductor film
leaving the two conductor pads uncovered. Next, five of the cut
ferrite pieces are punched with 0.125-inch diameter holes to
facilitate electrical contact with the buried inter-digitated
silver conductor. The five pieces of ferrite tape with holes are
aligned and stacked on top of the screen-printed and dried silver
conductor and the remaining four pieces of ferrite tape are aligned
and stacked on the other side, underneath the silver conductor.
This stack of aligned tapes is then laminated and fired to form a
multilayer test part comprised of the thick film buried silver
conductor sandwiched between five layers of the LTCC ferrite tape
on each side of it. The dielectric break down voltage is measured
using Clare Instruments Ltd. (Sussex, UK) Flash Tester, Model
A203D/213. The insulation resistance is measured using the
Hewlett-Packard High Resistance Meter, Model 4329A operating at
100VDC. The surge resistance of the multilayer test part is
measured using Compliance Design, Inc., Universal Surge Generator,
Model CDI-M5 equipped with a 2.times.10 .mu.s/5000V/1000A
wave-shape plug-in. This is a pass or fail test where in a 5000VDC
power surge is applied to the test part. This power surge has the
following wave-shape: rise or surge in 2 microseconds from 0 to
5000V peak open-circuit voltage followed by an exponential decay
characterized by a drop to half of the peak value, from 5000V to
2500V in 10 microseconds. An acceptable test part can withstand
this 5000VDC power surge.
In the materials system of this embodiment following factors have a
major influence on the dielectric characteristics of interest: (1)
Concentration of the sintering aid in the LTCC ferrite tape or the
thick film ferrite paste, (2) Peak firing temperature of the
multilayer package, (3) Concentration of solids in the thick film
buried silver conductor paste, (4) Use of a grain growth inhibitor
in the thick film buried silver conductor paste and, (5) Thickness
of the dielectric film covering the buried silver conductor.
Example 5
Evaluating Impact of the Sintering Aid
In the preferred embodiment of this invention presence of a
sintering aid in the ferrite helps densification of the ferrite
matrix at peak firing temperatures below 950.degree. C., essential
for use of buried silver conductors. With multilayer test parts
fired at peak firing temperature of 900.degree. C., data reported
in Table 2 shows that LTCC ferrite tape with more than 1% Bi.sub.2
O.sub.3 as sintering aid has higher breakdown voltage compared to
that for LTCC tapes with less than 1% of this sintering aid.
Example 6
Evaluating Impact of Peak Firing Temperature
With the materials system of this embodiment firing the LTCC
ferrite tape multilayer test part at a higher peak firing
temperature (Peak T) increases the magnetic permeability
(.mu..sub.m), dielectric breakdown voltage (BDV), insulation
resistance (IR) and surge resistance (SR). Data reported in Table 3
shows that the multilayer test parts fired at peak temperature of
930.degree. C. have higher .mu..sub.m BDV, IR and, SR than that for
multilayer test parts fired at peak temperature of 885.degree. C.
This improved performance with higher peak firing temperature is
most likely due to the higher degree of sintering and densification
in the multilayer package.
Example 7
Evaluating Impact of Variations in Buried Silver Conductor
Formulation
During high temperature processing of the multilayer package the
interaction between the buried silver conductor and the ferrite
results in diffusion of the silver into the ferrite matrix. The
extent of such silver diffusion can be controlled by different
means; one of them is lowering the amount of silver present in the
buried film by reducing the concentration of solids in the
conductor paste. Use of lower concentration of silver limits the
amount of silver available for interaction with the ferrite there
by potentially reducing any adverse effect such diffused silver may
have on the dielectric characteristics of interest. In the
materials system. of this embodiment reducing the concentration of
solids in the thick film buried silver conductor paste results in
an increase in dielectric breakdown voltage (BDV), insulation
resistance (IR) and surge resistance (SR). With multilayer test
parts fired at peak firing temperature of 930.degree. C., data
reported in Table 4 shows that parts fabricated with relatively
lower concentration of solids in the thick film buried silver
conductor paste have higher BDV, IR and, SR compared to that for
parts fabricated with a relatively higher concentration of solids
in the thick film buried silver conductor paste.
During high temperature processing of the multilayer package
presence of a grain growth inhibitor in the silver conductor paste
helps moderate activity of the silver particulates potentially
limiting any detrimental interaction between the silver and the
ferrite. In the materials system of this embodiment use of a grain
growth inhibitor in the thick film buried silver conductor paste
results in increased dielectric breakdown voltage (BDV), insulation
resistance (IR) and surge resistance (SR) . With multilayer test
parts fired at peak firing temperature of 930.degree. C., data
reported in Table 5 shows that parts fabricated with the thick film
buried silver conductor containing a grain growth inhibitor have
higher BDV, IR and, SR compared to that for parts fabricated with
the buried thick film silver conductor without any grain growth
inhibitor.
Example 8
Evaluating Impact of Variations in Thickness of Dielectric
Films
A thicker dielectric film offers greater resistance to electrical
breakdown. In the materials system of this embodiment with
multilayer test parts fired at peak temperature of 930.degree. C.,
data reported in Table 6 shows that a thicker dielectric film
increases the dielectric breakdown voltage (BDV) and data reported
in Table 7 shows that a thicker dielectric film increases the
insulation resistance (IR).
In the test part design of this embodiment use of a 1-mil thick
lower permeability dielectric to cover the buried inter-digitated
silver conductor improves the dielectric characteristics of the
multilayer test part; the dielectric breakdown voltage increases to
values greater than 2000V and the insulation resistance increases
to values greater than 1,000 M.OMEGA.. In miniature non wire-wound
multilayer transformers this beneficial effect of low permeability
dielectric film on top of the buried silver induction coil
manifests itself with enhanced dielectric breakdown voltage to
values greater than 1500V, comparable to conventional or
traditional wire-wound miniature transformers.
The observed performance characteristics for the multilayer test
parts and magnetic components are determined by materials related
variables such as, chemistry and particle size, and process related
variables such as peak firing temperature and soak-time. In the
preferred embodiment of this invention key materials and process
related variables are optimized so as to facilitate fabrication of
miniature multilayer magnetic components with magnetic coupling
coefficient greater than 0.95 and dielectric breakdown voltage
greater than 500V/mil at peak firing temperature below 950.degree.
C. using typical multilayer microelectronic manufacturing
techniques.
Those with expertise in this technology will recognize that that
further variations of the above are contemplated within the
disclosed invention.
TABLE 1 Magnetic Permeability, .mu..sub.m of LTCC ferrite tapes
Peak T Tape A Tape B Tape C Tape D Tape E 885.degree. C. 215 218
216 464 481 900.degree. C. 255 292 429 506 515 930.degree. C. 290
479 637 718 730 1030.degree. C. 440 689 761
TABLE 2 Concentration of Bi.sub.2 O.sub.3 in ferrite BDV less than
1% 2000 V greater than 1% 4000 V
TABLE 3 Peak T .mu..sub.m BDV IR SR 885.degree. C. 43 4000 V 1,000
M.OMEGA. Fail 5000 V 930.degree. C. 73 Above 5000 V 100,000
M.OMEGA. Pass 5000 V
TABLE 4 Relative concentration of solids in buried silver paste BDV
IR SR High 3100 V 1,000 M.OMEGA. Fail 5000 V High 3000 V 50,000
M.OMEGA. Fail 5000 V Low Above 5000 V 80,000 M.OMEGA. Pass, 5000 V
Low Above 5000 V 100,000 M.OMEGA. Pass 5000 V
TABLE 5 Type of buried silver paste BDV IR SR Without grain growth
inhibitor 3000 V 50 M.OMEGA. Fail 5000 V With grain growth
inhibitor Above 5000 V 80 M.OMEGA. Pass 5000 V
TABLE 6 Dielectric film thickness BDV 40 microns 1360 V 45 microns
1700 V 54 microns 3312 V 60 microns 4400 V
TABLE 7 Dielectric film thickness IR 25 microns 20,000 M.OMEGA. 41
microns 60,000 M.OMEGA. 66 microns 200,000 M.OMEGA.
* * * * *